Any way to design a 9 m (30 ft) high cantilever masonry fire wall? 2

[nivoo_boss]

Hey everyone!

So I have to solve a problem with a firewall between two buildings. I cannot tie it to the buildings on both sides so it has to be free-standing. I'm thinking about 240 mm CMU filled and reinforced blocks but even just with these the slenderness is about 2,8 times over the allowed slenderness by EC6.

What might be the solution here? Piers? How to design the piers - as RC columns perhaps? The support moment from wind is not too much, design moment is around 14 kNm/m at the base.

 

[phamENG]

Aesur, I don't think 706.2 applies here. 706.1.1 indicates that walls on the lot line between adjacent buildings "used or adapted for joint service between the two buildings" has to comply with 706. Otherwise, it would be 705. OP sounds like there is already a building next door (between two buildings), suggesting that it won't be joint service.

Exterior walls shall extend to the height required by Section 705.11. Interior structural elements that brace the exterior wall but that are not located within the plane of the exterior wall shall have the minimum fire-resistance rating required in Table 601 for that structural element. Structural elements that brace the exterior wall but are located outside of the exterior wall or within the plane of the exterior wall shall have the minimum fire-resistance rating required in Tables 601 and 602 for the exterior wall.

So it sounds to me like anything that provides stability to the wall needs to (theoretically) survive as long as the wall does. That makes sense.

Very few things in the code apart from live loads intend to completely prevent catastrophic collapse. It's all about mitigating the risk to an acceptable level. So we can never guarantee that the wall will stand just so or fall just so after a fire. We can design the building such that these critical elements won't lose stability within a reasonable amount of time during which first responders can mount an offense, hopefully get the fire under control, and evacuate people who are or could be impacted.

 

[phamENG]

Oh...just found the follow up post where they explained there's no building next door. So perhaps I'm wrong...

 

[jayrod12]

BA,

Are you truly telling me that you design every lot line wall on every building to free stand? Because that's what it sounds like.

I keep reiterating it without solid backup for a couple reasons:
1) I don't practice where the majority of members practice and have no desire to try referencing the IBC. I could try to find stuff in the NBCC for you, but I don't feel that the code is going to be that explicit "Thou shalt always ensure freestanding lot line walls".
2) As per 1, I don't believe the code is that explicit in most cases. It is explicit when the wall must serve both structures, but when it only serves a single structure I don't believe they are that explicit. Please feel free to point me to NBCC clauses where it indicates otherwise.
3) I have never, and granted I haven't practiced as long or as wide as you, been directed to do so. I'm no spring chicken in this game anymore. You'd think that the requirement would've come up on at least one job in the past 15 years, but it hasn't yet. Maybe I've just been lucky.

 

[BAretired]

Are you truly telling me that you design every lot line wall on every building to free stand? Because that's what it sounds like.

Definitely not! But I cannot remember ever having designed a three story wood framed building with full height exterior masonry wall touching the property line. In my view, this is a special case which should either be avoided or very carefully considered (preferably avoided).

 

[jayrod12]

Ah ok then. I was worried for some reason you and I were having a severe disagreement on expectations. It appears not.

That is a good point about wood framed, I don't believe the OP ever specifically said it was wood framed. I believe we all just surmised that with the whole "burnt down" part.

Where I practice, we aren't allowed to have combustible materials within the setback I don't believe, so I'd have to say I haven't designed a building like that either!

 

[BAretired]

Okay jayrod, looks as though we're on the same wavelength.

 

[JAE]

In a downtown setting there are millions of buildings with brick walls on the property line (i.e. facing the sidewalk/street where people walk, where the wall isn't a free standing wall.

As I mentioned above, there's a huge difference in the code between:

1. Fire demising walls required by the code to maintain the maximum area of building for the type of construction.
AND
2. Exterior walls of buildings that require a set level of fire resistance.

From the codes, Wall Type 1 above requires the designer to design the demising wall to remain standing if the structure on one side fails.

Wall Type 2 should very well be attached to the structure unless for some odd reason the designers want it free-standing (which rarely if ever happens).

 

[BAretired]

Thanks JAE, looks like I was mistaken.

So initially, this would be exterior Wall Type 2 which should be tied to the structure at each floor level and roof to stabilize the wall; the structure would require a minimum fire rating specified by code. If the minimum fire rating is attainable in a wood framed structure, and I'm guessing it is, the code would be satisfied.

If in the future, new construction is added beyond the wall for "joint service", the wall becomes demising Wall Type 1 which can be tied to the structure on both sides and again, the code is satisfied without making it free standing.

 

[JAE]

Well, in your new construction addition, the architect, or appropriate person, would typically do a code analysis and determine if the addition increases the building area beyond the code limits. If it doesn't, no demising wall is needed. If it does, then the old exterior wall becomes a demising wall, as you stated. I'm not sure how the "remain standing" then applies unless the new addition has its own wall, directly adjacent to the original exterior wall.

 

[80PercentTruth]

Design the exterior firewall freestanding and switch from CMU to concrete construction. CMU is not the right building material for this application in my view. I've designed many firewalls to provide line of sight separation (NFPA 850) between heavy duty transformers in coal fired power plants. Usually the design results in 12-18" thick walls up to 30' in height.

"Engineers only know about 80% of the truth, the next 10% is very difficult to achieve, and the last 10% impossible. If we are bound to be wrong, we may as well be wrong simply and conservatively."